16-mm sound motion pictures : a manual for the professional and the amateur (1953)

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274 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT The Clarkstan Corporation is also marketing a signal generator that generates the same form of signal as that produced with the Clarkstan records, thereby eliminating the phonograph and pickup as a source of disturbance in the test signal. Photographs of oscilloscope traces obtained in testing a pickup, and an explanation of the significance of the traces can be found in Audio Engineering, October 1947, p. 18, "Analyzing Sweep Frequency Transcriptions," by Wayne R. Johnson. It must be admitted quite frankly that commercial distortion-measuring equipment is by no means adequate for channel checking, since testing is very laborious and time-consuming and the parameters that are checked are too few, and not related in a sufficiently close manner to the distortion produced by the system when speech, music, or other sounds are recorded. For this reason the most practical test is the recording of test material (such as unrelated words, sounds, noises, etc.) that are similar to some of the sounds that the system is expected to record. Words with sibilants and consonants are most useful for checkvariable-area systems ; sounds such as piano notes are useful for checking wows and lower speed flutter ; and sounds such as violin notes are useful in checking higher speed flutter. Unfortunately the results of such tests are difficult to evaluate numerically. Among manufacturers of test equipment, there are numerous "overlappings" of apparatus; but, since the design objectives of different manufacturers vary in detail, it is necessary to determine just what a particular instrument was designed to measure in order to interpret the numerical readings obtained from it. For this reason it is rare that the instruments of two different manufacturers of the same apparent type will give equal readings; the differences in readings reflect differences in instrument design quite as much as differences in performance. Since all of these instruments use some form of artificially produced tone which cannot be an accurate simulation of whatever sound is to be recorded, the significance of one instrument in one case may be better than the significance of another; in another case, when music rather than speech is recorded, the reverse may be true. While it may seem trite, it is necessary to repeat that the best test for a system intended to record sound is to record the intended or similar sound. Unfortunately no specific kind of artificially produced tone can accurately simulate all kinds of natural sounds that are to be recorded. Unfortunately, too, the distortion products of practical systems such as variable area, variable density, disk recording, magnetic recording, etc., are quite different in